Generally, a discharge lamp lighting apparatus is provided with an inverter and an LC resonator. At the time of starting a discharge lamp, the inverter is driven at a very high frequency. A high-frequency AC power obtained by the operation of the inverter is impressed to a discharge lamp via an LC resonator. The frequency of the AC power output is then gradually lowered close to a resonance frequency of the LC resonator. Thereby, the operation frequency of the inverter matches with the resonance frequency of the LC resonator, and the voltage impressed to a discharge lamp rises to maximum. When the impressed voltage reaches a glow-discharge starting voltage of the discharge lamp in the course of the output frequency of the LC resonator closing to the resonance frequency of the LC resonator, the discharge lamp starts a glow discharge. After that, when the output frequency reaches the resonance frequency of the LC resonator, the glow discharge transfers to an arc discharge, and thus the discharge lamp is lit up. After the discharge lamp has been lit up, the frequency of the inverter is further lowered so as that the discharge lamp is steadily lit up at a frequency not involving acoustic resonance in the discharge lamp.
On the other hand, the applied voltage must be kept high for an adequate time to ensure the glow discharge to arc discharge transition of the high-pressure discharge lamp. A prior art, Japanese laied-open patent application JP2000-58284, discloses discharge lamp ballast having a booster provided in preceding the inverter. In a starting operation, the booster boosts up the applied voltage higher than a steady lighting voltage for starting the operation of the high-pressure discharge lamp. During that the high-pressure discharge lamp is steadily lighting, lamp power is adjusted near a rated power by the output voltage of the booster.
There exists an astable operation window, i.e., a frequency zone where an acoustic resonance occurs in the high-pressure discharge lamp. Thereby, at the time of starting as well as during a steady lighting state, the high-pressure discharge lamp is started and lit up at a frequency in a stable operation window, i.e., a frequency zone where an acoustic resonance never occurs in the high-pressure discharge lamp. Another prior art, Japanese laied-open patent application JP2002-42732, discloses a high-pressure discharge lamp having an arc tube with a sphericity of 0.53 to 0.84, an inner diameter of 2.0-6.0 mm, and a plurality of stable operation windows, i.e., frequency zones which are free from causing acoustic resonance.
By the way, at the time of starting the high-pressure discharge lamp it is necessary to impress voltage higher than a steady lighting voltage. It is also necessary to keep a lamp voltage around the rated power of the lamp by a booster during the steady lighting state of the high-pressure discharge lamp. Therefore, there are problems increasing in cost and size, correspondingly.